1. Field of the Invention
The invention relates to x-ray scanners and more particularly to a method for calibrating devices of this type which makes use of a single non-circular standard and to a system for the application of said method.
2. Description of the Prior Art
In order to examine a patient, it is becoming an increasingly common practice to make use of x-ray devices known as scanners which produce images of cross-sections of the patient. These devices are based on the physical phenomenon of absorption of x-rays by the human body. This absorption is directly related to the distance x traveled by x-rays within the body in accordance with the formula: EQU I=I.sub.o e.sup.-bx
where:
I.sub.o is the intensity of radiation entering the human body PA1 I is the intensity of radiation emerging from the human body, PA1 b is a coefficient of attenuation which depends on the body being traversed. PA1 a) positioning of the standard between the x-radiation source and the detection device ; PA1 b) measurement of attenuations in the N channels of the detection device in respect of P principal angular positions of the rotating structure and in respect of n elementary positions in close proximity to each other about each principal angular position ; PA1 c) computation of the mean value of the n attenuations in respect of each channel and in respect of each of the P principal angular positions ; PA1 d) smoothing of the N mean values of attenuations from one channel to the next in respect of each of P principal angular positions so as to obtain a curve of response of attenuation as a function of the position of the channel in which the high-frequency components have been eliminated, PA1 first means for computing the logarithm of the N signals delivered by the N channels, PA1 second means for computing in the case of each channel the attenuation introduced by the standard, PA1 third means for computing in the case of each channel the mean value of the n measured attenuations in respect of a given principal angular position, PA1 fourth means for computing the Fourier transform of the N mean attenuations corresponding to a principal angular position, PA1 fifth means for eliminating the high-frequency components in the spectrum resulting from the Fourier transform, PA1 sixth means for computing the inverse Fourier transform of the low-frequency components of the spectrum, PA1 seventh means for computing a polynomial approximation of the curve resulting from the inverse Fourier transform, PA1 and means for retaining in memory the values representing the polynomial approximation of each response curve relative to a principal angular position.
In a logarithmic measurement scale, the attenuation I/I.sub.o is equal to bx or in other words is proportional to the distance x.
As shown in FIG. 1, these devices are essentially constituted by an x-ray source 10 associated with a detection device 11, these two elements being disposed in a fixed geometrical relationship with respect to each other in such a manner as to ensure that the body to be examined can be interposed between them. In addition, they are supported by a structure (not shown in the drawings) which is capable of rotating about the body to be examined so as to irradiate the body at different angles. The x-ray source which is controlled by a device 13 emits its rays in an angular sector which is of sufficient width to illuminate the entire cross-section of the body. The detection device 11 has the shape of an annular sector, the length of which is adapted to the width of the x-ray beam and is constituted by a large number of elementary detectors 12 in juxtaposed relation.
In order to obtain an image of the cross-section of the human body traversed by the x-ray beam, the structure which supports the source 10 and the detection device 11 is displaced in rotation about the body and the output signals of the elementary detectors 12 are measured for suitable processing in accordance with known methods in order to obtain an image which is representative of the cross-section. For this treatment, the elementary detectors 12 (also known as channels) are connected to an electronic device 14 which first computes the logarithm of the signals received so as to obtain a signal whose amplitude is proportional to the attenuation of the x-rays.
As a result of different phenomena which will not be explained here, the amplitude of the aforementioned signal in the case of each elementary detector or channel is not proportional to the attenuation which has in fact been sustained. In consequence, in order to remedy this drawback, consideration has been given to various methods which consist for example in recording the output signals of the channels in the presence of bodies having known dimensions and a known coefficient of absorption in order to compute the attenuations (logarithm calculations) and to compare these measured attenuations with values computed as a function of the dimensions and of the absorption coefficient of the body or standard. These comparisons make it possible to deduce a law of correspondence or a modifying law between the measured values and the values which should be obtained. This law can be in the form of correspondence files or of mathematical formulae representing this correspondence in respect of each detection channel.
By way of example, the standards which are employed for performing these so-called calibration measurements are shims having different thicknesses which are introduced in proximity to the x-ray source, thus entailing the need for handling operations at the level of the source in order to insert and remove said shims. Furthermore, the shape of these shims and their position are far removed from the shape and position of the body of the patient to be examined, thus increasing the non-linearity of the system.
In U.S. Pat. No. 4,352,020, it is proposed to employ circular shims 15 to 17 having different diameters which are disposed at the center of rotation of the support structure. This makes it possible to come closer to the conditions of measurements which will be made on the body to be examined. This patent also proposes to make use of a standard in the form of a circular-section cone which is displaced transversely with respect to the beam so as to obtain different lengths of attenuation. With the standards described, the measurements are performed in respect of a predetermined position of the support structure and in the case of each standard.
FIG. 2 shows the shape of three response curves 20, 21 and 22 of attenuation as a function of the position of the channels in the case of measurements on three standards of circular shape. The measured values are represented by the dots and vary in the vicinity of a mean value which represents the theoretical value in a linear system. These curves can be employed as follows : when the measured signal corresponds to a point A, it will be deduced therefrom that the linear signal is the point A' of the mean curve 20. When the measured signal corresponds to a point B located between the curves 20 and 21, the linear signal will be deduced therefrom by interpolation between the curves 20 and 21. This interpolation can be computed in accordance with a linear law or more generally a polynomial law.
The curves 23 and 24 of FIG. 3 show in a different form the principle of calibration at the level of a channel. These curves describe within a given channel the attenuation as a function of the thickness x in the case of measured values (curve 23) and in the case of computed values (straight line 24). In fact, the measured values give points which are joined to each other in accordance with a predetermined law, namely either linear or polynomial, so as to obtain a continuous curve. When measuring an attenuation, this corresponds for example to point C of curve 23 and there is deduced therefrom the linear value corresponding to point C' of curve 24.
The U.S. patent cited earlier describes a device in which the correspondence between the measured values and the real values of attenuation is effected by a system of files created during the calibration operation. In regard to interpolation, the patent proposes linear, cubic and biquadratic interpolations but only the linear interpolation is described in detail.
The methods of calibration which have been briefly described in the foregoing suffer from a major disadvantage in that they call for the use of a number of standards, thus involving a large number of handling operations. Moreover, these handling operations have to be accurate, especially in the case of circular standards, the different centers of which must coincide with the center of rotation of the structure.
It is worthy of note that the U.S. patent proposes to employ a single standard which would have the shape given by FIG. 12 of said patent and to rotate the structure about said standard, thereby obtaining absorption paths of different lengths according to the angular position of the structure. However, this mode of operation is only mentioned and does not indicate either the method or the means for applying the method in this case.